Electrical conduction and electroluminescence of vacuum-evaporated thin TCNQ films

Electroluminescence (EL) and electrical conduction in TCNQ (tetracyanoquinodimethane) thin films prepared by the vacuum-evaporation technique were investigated. X-ray diffraction studies showed that the specimen prepared at 280°C was crystalline, while the one prepared at 180°C was amorphous. An infrared (IR) absorption analysis showed that the main chemical composition of TCNQ has been largely preserved in the vacuum-evaporated films. The polarized IR spectrum indicated that the aromatic ring of the evaporated TCNQ molecules is arranged nearly parallel to the substrate. The electrical properties depended strongly on the film deposition conditions, such as deposition rate and/or temperature and substrate temperature. The conduction currents were also affected by the electrode materials. Higher conductivity was observed in the specimen with a Cu anode than that with Au or Al. A weak EL which was well in proportion to the conduction current was observed in the high-field region of >10⁵ V/cm. The thermally stimulated current showed two peaks at -120 and -20°C. The trap depths were estimated as 0.10 and 0.22 eV, respectively